Murphy's Design Principle can be stated simply as
If something can go wrong, it will.,
and unfortunately, it became one of its own first victims.
Murphy intended this principle as an aid for designers, the idea being that engineers should look for ways in which their projects can go wrong, and then use that principle to discover how to change the design so that going wrong isn't possible.
The traditional example is provided by the electrical connectors
for aircraft flight controls.
A wiring harness leads from each wing to the cockpit,
where it plugs into one of two sockets in the instrument panel.
The two plugs are identical, but are coloured red and green,
as well as
to match the corresponding markings on the sockets.
This is to make it extremely obvious which plug should go into
which socket, as switching them would be disastrous.
Murphy argued that it doesn't matter how obvious the right way is, eventually someone, somewhere, will do it the wrong way. The only correct design would be to make it physically impossible for a plug to be connected to the wrong socket, such as by using different pin shapes or arrangements.
You should carefully examine every project design for ways
that the finished product can be used incorrectly,
and should change the design so that it is no longer possible
for anyone to use it incorrectly.
It doesn't matter how many warning labels there are,
nor how many times the instruction manual says
Don't do this!, because eventually someone will do it.
This principle was originally called
and almost immediately it became popular,
not as a way of preventing problems, but of explaining them.
accidents, and even disasters could then be
humorously passed over by blaming them on
Calling it a law, rather than naming it explicitly as a design principle,
allowed people to use it incorrectly, and ironically, as predicted,
since they could use it incorrectly, they did.